1. Field of the Invention
The present invention relates to a method for the heat treatment of products contained in sealed containers, by exchange between a fluid and said products, in particular to effect the sterilization, with or without back-pressure or excess pressure, of food, pharmaceutical or the like products in the most diverse of sealed wrappings, such as metal cans, sachets, boats, cups, of aluminum or of plastics material, or even of composite materials comprising these different elements, or also such as glass containers, etc., hereinafter sometimes referred to as sealed packages.
The present invention also relates to a device for applying this method.
2. Description of the Prior Art
It is known to use, notably for the sterilization of food products, closed or open enclosures, in particular autoclaves, which permit high pressures and temperatures, higher than 100.degree. C, which utilize either a liquid fluid such as water, in which the products to be sterilized are immersed, or a gaseous fluid such as steam or a suitable steam-air or steam-gas mixture. These various methods permit the enclosure to be brought to a temperature and to a pressure, more or less considerable, independant of one another.
In the autoclaves in which the rise in the temperature of sterilization and the cooling are effected by using a liquid fluid such as water, for example, in which the containers whose contents must be sterilized are entirely immersed, the rise in temperature is produced by direct or indirect heating (for example by heating by the direct injection of steam into the water, by heating by indirect action, by means of coils for example, in which a fluid circulates, by electrical resistance heating, etc.). The cooling may be effected by the same means where cooling the hot water indirectly is concerned, or the more or less sudden replacement of the hot water with cold water.
If necessary, the final phase of cooling can be done by sprinkling with cold water.
The back-pressure or excess pressure necessary in certain cases is exerted by a gas injected above the level of the fluid in the enclosure, this gas being generally compressed air.
The main drawbacks of this method are the following:
(a) a very high consumption of water,
(b) a fairly considerable loss of heat, difficult to recover, the water being polluted,
(c) more or less heterogeneity of the temperatures inside the batch of the containers,
(d) a speed of thermal exchange between the ambient fluid and the containers which is relatively low.
It has been sought to offset the latter drawbacks by stirring the water around the containers, which permits acceleration of the heat exchanges and improvement of the homogeneity of the temperature inside of the batch. This agitation is effected generally by pumping, that is to say by recirculating the heating or cooling water by sucking it in at the base of the enclosure and reinjecting it thereinto at a higher level.
Autoclaves which use live steam as the heating fluid have the advantage of ensuring a better rise in temperature, that is to say more rapid, and an economy of water in the course of sterilization, this water being represented essentially by condensates produced during the sterilization and the cooling.
Such a thermal exchange has, however; the drawback of being a too slow because the steam contains all the greater quantity of air. The purging problems are hence very considerable. Certain methods tend to accelerate the heat exchange in the course of rise in temperature and in the sterilization phase, in particular by more or less intensive stirring of the air-steam mixtures by an internal recirculating system for the heating fluid through the batch of containers, by a turbine or if necessary by any other means of similar efficiency.
However, heating with steam has the following drawbacks:
(a) homogeneous temperature in the whole batch is difficult to achieve, even with vigorous stirring. This stirring leads to the creation of preferential paths in the batch of the containers, which preferential paths cause certain containers to benefit from a better heat exchange than those which are outside these preferential paths. Hence in general, steam or gas-steam mixtures act like water, as regards the homogeneity of the temperatures;
(b) the regulation of the excess pressure is difficult to accurately control because the injected gas, generally air, which is used to ensure the excess pressure, has a considerable coefficient of expansion. Now, certain packages are very sensitive to even very low variations in pressure in the course of the treatment;
(c) the passage from the sterilization phase to that of cooling poses delicate problems; in particular, if the cooling is done in air, by gradual condensation of the steam and by a suitable means, this cooling has proved to be extremely time-consuming, the thermal exchange between the air, even agitated, and the walls of the containers being very low and very slow.
Hence in general, water cooling has been substituted for air cooling, either by completely immersing the containers contained in the enclosure, which are until now in an environment of steam or of a steam-air or a steam-gas mixture, or by sprinkling the batch of containers with cold water.
This system has the drawback of being subjected to very great suddenness in pressure drop due to the sudden condensation of the steam and in spite of a considerable and simultaneous injection of gas (generally compressed air). The maintenance of the excess pressure applied at the end of the sterilization phase being indispensable in the case of packages sensitive to variation in pressure, it follows that considerable risks exist of the partial or total destruction of these packages. This drawback is generally offset by compensating methods. However the latter are still difficult to apply and of little reliability. In any case they do not give a sufficient mastery of the variations in pressure on the passage from the sterilization phase to the cooling phase;
(d) finally, cooling by uncontrolled sprinkling involves a fairly considerable heterogeneity of the temperatures which exist in the midst of the batch of sealed containers containing the products.
Moreover, whatever the autoclave system concerned, the consumption of energy, either electrical, (circulation or stirring), or thermal (in particular in autoclaves using a liquid fluid such as water), or in the form of a supplementary gas to maintain a pressure (such as compressed air, nitrogen, etc..) is considerable.